Apparatus and method for Polarization Mode Dispersion Compensation (PMDC) and Chromatic Dispersion Compensation (CDC)

ABSTRACT

(Polarization Mode Dispersion Compensation) and CDC (Chromatic Dispersion Compensation) in an optical signal are provided. An optical signal has a first polarization and a second polarization that lags the first polarization with a DGD (Differential Group Delay), Δτ. The first and second polarizations are each aligned with a respective one of slow and fast principal axes of a PM (Polarization Maintaining) fiber having a chirped grating. The first and second polarizations propagate through the PM fiber and are reflected at two different points along the PM fiber in a manner that the first and second polarizations emerge from the PM fiber in synchronization after being reflected. In some embodiments, the optical signal then propagates through an optical fiber having a chirped grating to control dispersion.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. ______ filed Feb. 28, 2002.

FIELD OF THE INVENTION

[0002] The invention relates to dispersion in optical transmissionsystems. More particularly, invention relates to polarization modedispersion compensation and chromatic dispersion compensation.

BACKGROUND OF THE INVENTION

[0003] When an optical signal propagates through an optical transmissionmedium such as an optical fiber chromatic mode dispersion andpolarization mode dispersion occurs over distances. More particularly,in PMD (Polarization Mode Dispersion), two polarizations of the opticalsignal, each aligned with one of two principal axes of polarization ofthe optical fiber, have different group velocities. The different groupvelocities of the polarizations result in a PGD (Polarization GroupDelay) which, in turn, results in PMD. Some techniques have been used toperform PMDC (Polarization Group Delay Compensation) in OC192 systems bycontrolling the polarizations using an external polarization controllerand having the optical signal propagate through a fixed length of PM(Polarization Maintaining) fiber. The external polarization controllermanipulates the two polarizations of the optical signal in a manner thatone of the two polarizations having a faster group velocity is rotatedparallel to a slow principal axis of the PM fiber and that the other oneof the two polarizations having a slower group velocity is rotatedparallel to a fast principal axis of the PM fiber resulting in areduction in the DUD and mitigation of the PMD as the optical signalpropagates through the PM fiber. Since the PM fiber has a fixed length,the reduction in the DGD is constant. As such, when DGD of the opticalsignal, at an input of the polarization converter, varies in time, thePM fiber can only partially compensate for the DGD. Therefore, the PMfiber can only partially compensate for PMD.

SUMMARY OF THE INVENTION

[0004] Methods and Apparatuses for Performing PMDC

[0005] (Polarization Mode Dispersion Compensation) and CDC (ChromaticDispersion Compensation) are provided. An optical signal has a firstpolarization arid a second polarization that lags the first polarizationwith a DGD (Differential Group Delay), Δτ, resulting in PMD(Polarization Mode Dispersion). Furthermore, in some cases the opticalsignal is dispersive resulting in CD (Chromatic Dispersion). The PMD andthe CD degrade the quality of the optical signal and limit the distanceat which the signal can propagate before having to be regenerated.

[0006] The first and second polarizations are each aligned with arespective one of a slow principal axis and a fast principal axis of abirefringent wave-guide with chirped grating. The birefringentwave-guide may be a PM (Polarization Maintaining) fiber. The first andsecond polarizations enter the birefringent wave-guide sequentially intime and are reflected at different points along the birefringentwave-guide due to coupling with the chirped grating. An additional DGD,Δτ′, is introduced in the birefringent wave-guide with grating due todifferent group velocities of the first and second polarizations and dueto different distances traveled by the first and second polarizations inthe birefringent wave-guide with chirped grating. The additional DGD,Δτ′, offsets the DGD, Δτ, so that the first and second polarizationsemerge from the birefringent wave-guide with chirped grating insynchronization with a total DGD, Δτ₁=Δτ+Δτ′=0. This results in PMDC, Incases where the optical signal is dispersive, the optical signal thenpropagates through an optical wave-guide with chirped grating. In such acase, a continuous range of wavelengths, λ_(j), enter the opticalwave-guide sequentially in time and are reflected, due to coupling withthe chirped grating, at different points along the optical wave-guide ina manner that the wavelengths, λ_(j), emerge from the optical wave-guidein synchronization. This results in CDC.

[0007] Embodiments of the invention are not limited to the speed oftransmission, however, they are particularly useful for optical systemstransmitting information at 40 Gbps and beyond where PMD and CD can bothrestrict the length over which a signal can be transmitted before havingto be regenerated. Therefore, embodiments of the invention enable PMDCand CDC in long fiber transmission links for high-speed opticaltransmission. Providing PMDC and CDC in such links reduces the number ofrequired regeneration sites and therefore reduces link costs.

[0008] In accordance with a first broad aspect of the invention,provided is a an optical apparatus. The apparatus has a birefringentwave-guide which is used to receive an optical signal having a firstpolarization and a second polarization. The birefringent wave-guide maybe a birefringent planar wave-guide or may be a PM fiber. The apparatusis also used to allow the first polarization and the second polarizationto propagate at different group velocities, The birefringent wave-guidehas a chirped grating which is used to reflect the first polarizationand the second polarization at different points along the birefringentwave-guide.

[0009] In accordance with another embodiment of the invention, providedis an optical apparatus adapted to perform PMDC. The apparatus has abirefringent wave-guide comprising a fast principal axis, a slowprincipal axis and a chirped grating. The apparatus also has a PC(Polarization Controller) connected to the birefringent wave-guide. ThePC receives an optical signal and aligns a first polarization to one ofthe slow and fast principal axes. The PC also aligns a secondpolarization to another one of the slow and fast principal axes, whereinthe second polarization lags the first polarization with a DGD, Δτ. Thefirst and second polarizations are each aligned with a respective one ofthe slow and fast principal axes so that they will propagate atdifferent group velocities through the birefringent wave-guide and bereflected, through coupling with the chirped grating, at differentpoints along the birefringent wave-guide. The different group velocitiesand reflection at the different points result in the first polarizationundergoing a greater time delay in the birefringent wave-guide whencompared to a time delay, in the birefringent wave-guide, of the secondpolarization.

[0010] In some embodiments of the invention, the birefringent wave-guideis a PM fiber.

[0011] In some embodiments the optical apparatus may have an opticalcirculator connected between the PC and an input. The optical circulatormay be a 3-port optical circulator and it may be used to re-direct theoptical signal propagating from the input into the PC anti re-direct theoptical signal propagating from the PC to an output. Furthermore, insome embodiments, the optical circulator may be a chip opticalciculator.

[0012] The optical apparatus may be adapted to perform PMDC and CDC of adispersive optical signal having wavelengths, λ_(j), and an average DGD,<Δτ>. Such an apparatus may have an optical wave-guide with a chirpedgrating connected to the optical circulator. The optical wave-guide maybe an optical fiber or any suitable optically transmitting materialcapable of performing wave-guide functionality. Furthermore the opticalwave-guide may be integrated on a chip. The optical wave-guide may beused to receive the dispersive optical signal and reflect thewavelengths, λ_(j), at different points along the optical wave-guide ina manner that the wavelengths, λ_(j), emerge from the optical wave-guidein synchronization.

[0013] In accordance with another embodiment of the invention, providedis an optical apparatus used to perform PMDC. The apparatus has abirefringent wave-guide comprising a fast principal axis, a slowprincipal axis and a chirped grating. The apparatus also has an opticalcirculator and a PC. The PC is connected to the birefringent wave-guide,through the optical circulator. The PC is used to receive an opticalsignal and align a first polarization with one of the slow and fastprincipal axes. The PC also aligns a second polarization, which lags thefirst polarization with a DGD Δτ, to another one of the slow and fastprincipal axes. The first and second polarizations are aligned so thatthey propagate at different group velocities through the birefringentwave-guide and are reflected, through coupling with the chirped grating,at different points along the birefringent wave-guide. This results inthe first polarization undergoing a greater time delay in thebirefringent wave-guide when compared to a time delay, in thebirefringent wave-guide, of the second polarization.

[0014] The birefringent wave-guide may be a PM fiber or any suitablebirefringent material capable of transmitting the optical signal andcapable of performing wave-guide functionality. The birefringentwave-guide may also be integrated on a chip.

[0015] The chirped grating may have a spatial period, Λ, that varieslinearly or non-linearly along the length of the birefringentwave-guide. More particularly, in some embodiments the spatial period,Λ, may vary quadratically.

[0016] The birefringent wave-guide may be embedded in a piezo-electricdevice which may be used to stretch the birefringent wave-guide tocontrol the spatial period, Λ, of the chirped grating. An optical tapmay be connected to the optical circulator at an output to re-direct aminor portion of the optical signal to a control circuit. The controlcircuit may be used to detect a total DGD, Δτ₁=Δτ+Δτ′, of the first andsecond polarization from the minor portion of the optical signal,wherein Δτ′ is a DGD introduced in the birefringent wave-guide. Thecontrol circuit may provide instructions to the piezo-electric devicefor tuning the spatial period, Λ, of the chirped grating based on thetotal DGD, Δτ₁. The control circuit may also be used to measure apolarization state of the minor portion of the optical signal andprovide instructions to the PC for tuning an alignment of the first andsecond polarizations with a respective one of the slow and fastprincipal axes of the birefringent wave-guide, based on the polarizationstate. In other embodiments of the invention, the birefringentwave-guide may be embedded in one or more heaters. The heaters may beused to control effective indexes of refraction, n_(s,eff), n_(f,eff),of the slow and fast principal axes, respectively. Furthermore, thecontrol Circuit may provide instructions to the heaters for tuning theeffective indexes of refraction, n_(s,eff), n_(j,eff), based on thetotal DGD, Δτ₁.

[0017] In some embodiments another birefringent wave-guide may be usedto connect the optical circulator and the PC. Furthermore, the opticalcirculator may be a 3-port optical circulator.

[0018] An optical wave-guide having a chirped grating may also beconnected to the optical circulator so that the optical apparatus mayperform CDC in addition to PMDC. The optical wave-guide may be anoptical fiber or any suitable material capable of transmitting light andperforming wave-guide functionality. Furthermore, the optical wave-guidemay be integrated on a chip. The optical apparatus may be applied to adispersive optical signal having wavelengths, λ_(j), and an average DGD,<Δτ>. After the dispersive optical signal has undergone PMDC, theoptical wave-guide may be used to receive the dispersive optical signaland reflect the wavelengths, λ_(j), at different points along theoptical wave-guide in a manner that the wavelengths, λ_(j), emerge fromthe optical wave-guide in synchronization. In such embodiments, theoptical circulator may be a 4-port optical circulator. Furthermore, theoptical apparatus may have control means for tuning a dispersion of theoptical signal detected at an output. More particularly, the opticalwave-guide may be embedded in a piezo-electric device. The controlcircuit may detect a dispersion of a minor portion of the dispersiveoptical signal, at an output. The control circuit may then providedinstructions to the piezo-electric device, in which the opticalwave-guide is embedded, for tuning a spatial period, Λ′, of the opticalwave-guide, based on the dispersion, by applying a tensile force uponthe optical wave-guide and stretch the optical wave-guide. In otherembodiments of the invention, the optical wave-guide may be embedded inone or more heaters and the control circuit may provide instructions tothe heaters for applying heat to the optical wave-guide, based on thedispersion, to tune an effective index of refraction n′_(eff) of theoptical wave-guide and control the dispersion.

[0019] The functionality of the birefringent wave-guide, the opticalwave-guide, the PC, the control circuit and the optical tap may beintegrated on a chip. Furthermore, the PMD compensator and the PMD andCD compensator may be implemented in any optical transmission system.

[0020] In accordance with another embodiment of the invention, providedis a method of performing PMDC upon an optical signal having a firstpolarization and a second polarization. The second polarization lags thefirst polarization with a DGD, Δτ. The first polarization is alignedwith one of a slow principal axis and a fast principal axis of abirefringent wave-guide having a chirped grating and the secondpolarization is aligned with another one of the slow and fast principalaxes of the birefringent wave-guide. The first and second polarizationsare then propagated through the birefringent wave-guide at differentgroup velocities and reflected at different points along thebirefringent wave-guide. The first polarization and the secondpolarization are each aligned with a respective one of the slow and fastprincipal axes in a manner that the first polarization undergoes agreater time delay in the birefringent wave-guide when compared to atime delay, in the birefringent wave-guide, of the second polarization.

[0021] The method may be used to perform CDC in addition to PMDC, Insuch a method a dispersive optical signal has wavelengths, λ_(j), and anaverage DGD, <Δτ>. After the dispersive optical signal has undergonePMDC, the first and second polarizations are propagated through anoptical wave-guide with the wavelengths, λ_(j), entering the opticalwave-guide sequentially in time. In the optical wave-guide, thewavelengths, λ_(j), are reflected at different points along the opticalwave-guide in a manner that the wavelengths, λ_(j), emerge from theoptical wave-guide in synchronization after being reflected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

[0023]FIG. 1A is a schematic block diagram of a PMD (Polarization ModeDispersion) compensator, in an embodiment of the invention;

[0024]FIG. 1B is a side view of a PM fiber with grating of the PMDcompensator of FIG. 1A;

[0025]FIG. 2 is a schematic block diagram of a PMD compensator, inanother embodiment of the invention;

[0026]FIG. 3 is a schematic block diagram of a PMD compensator, inanother embodiment of the invention;

[0027]FIG. 4 is a schematic block diagram of a PMD compensator, inanother embodiment of the invention;

[0028]FIG. 5 is a schematic block diagram of a PMD and CD (ChromaticDispersion) compensator, in another embodiment of the invention;

[0029]FIG. 6 is a schematic block diagram of a PMD and CD compensator,in another embodiment of the invention;

[0030]FIG. 7 is a schematic block diagram of a PMD and CD compensator,in another embodiment of the invention;

[0031]FIG. 8 is a schematic block diagram of a PMD and CD compensator,in another embodiment of the intention; and

[0032]FIG. 9 is a schematic block diagram of a PMD and CD compensator,in yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In optical systems, optical fibers used as transmission mediahave imperfections such as geometrical asymmetries, doping asymmetries,asymmetrical stress and environmental variations. These imperfectionsresult in the optical fibers having a fast and a slow principal axis ofpolarization. When an optical signal propagates through such opticalfibers a component (referred to as a fast principal state ofpolarization) of the optical signal propagating along the fast principalaxis has a faster group velocity than that of a component (refereed toas a slow principal state of polarization) of the optical signalpropagating along the slow principal axis. As the optical signalpropagates through the optical fiber the difference in group velocitiescauses a DGD (Differential Group Delay) between the fast and the slowprincipal states of polarization wherein signal propagation with theslow principal state of polarization lags behind signal propagation withthe fast principal state of polarization. The DGD is typically measuredin picoseconds. In an optical fiber, the direction of the fast and theslow principal axes changes along the length of the optical fiberdepending on the imperfections. This results in a DGD that develops overdistances. Furthermore, a dispersive optical signal having a continuousrange of wavelengths, λ_(j), has a DGD associated with each one of thewavelengths, λ_(j), resulting in an average DGD, <Δτ>which follows aMaxwellian distribution as a function of distance of propagation. Moreparticularly, the average DGD <Δτ>∝{square root}{square root over (d)}where d is the distance travelled by the dispersive optical signalthrough the optical fiber. The average DGD develops over distances ofpropagation and results in PMD (Polarization Mode Dispersion). The PMDdegrades the quality of the optical signal. This effect is present atall transmission speeds however it is of particular importance inoptical systems of high bit-rate where the bit rate is as high as 40Gps.

[0034] Dispersion effects are also important in optical transmissionsystems. In dispersive media such as an optical fiber, the groupvelocity of an optical signal propagating through the optical fiberdepends on wavelength within a channel bandwidth of the optical signal.Optical wavelengths within a channel bandwidth travel with differentgroup velocities and this results in pulse broadening during propagationthrough the optical fiber. This pulse broadening results in inter-bitinterference and in an increase in a BER (Bit Error Rate). The increasedBER, in turn, limits the distance at which the optical signal canpropagate through the optical fiber before having to be regenerated.

[0035] Referring to FIG. 1A, shown is a schematic block diagram of a PMDcompensator 100, in an embodiment of the invention. The PMD compensator100 has a PC (Polarization Controller) 110, a 3-port optical circulator120 and a PM (Polarization Maintaining) fiber 130 with a chirped grating140. The PC 110 is connected to the PM fiber 130 through another PMfiber 150 and the 3-port optical circulator 120. More particularly, anend 102 of the PM fiber 130 is connected to the 3-port opticalcirculator 120. The two PM fibers 130, 150 are aligned so thatrespective fast principal axes of the two PM fibers 130, 150 are alignedand so that respective slow principal axes of the two PM fibers 130, 150are aligned. An optical fiber 160 is connected to the PC 110 and formsan input 170. An optical fiber 180 is connected to the PC 110 and formsan output 190.

[0036] The chirped grating 140 has a negative chirp in which a spatialperiod, Λ, decreases from the end 102 along its length from Λ_(a) downto Λ_(b), as shown in FIG. 1B. More particularly, the spatial period, Λ,decreases quadratically along the length of the PM fiber 130. In someembodiments of the invention, the chirped grating 140 of the PM fiber130 is formed by irradiating the PM fiber 130 with, for example,radiation at 244 nm through a phase mask which forms fringes within acore of the PM fiber 130. This modifies a refractive index in the coreby an amount that depends on the flux incident on the PM fiber 130.Modification of the refractive index in the core is caused by one of atleast two mechanisms. In one embodiment of the invention, the PM fiber130 is made of glass and is intrinsically photosensitive due to dopant,such as for example germanium or boron, incorporated within the glass.In another embodiment of the invention, the PM fiber 130 is sensitizedto make it photosensitive. More particularly, for example, the PM fiber130 is hydrogenated by being left in a vessel of high pressure hydrogenfor a period of time on the order of days to weeks. This allows thehydrogen to diffuse in the core of the PM fiber 130 making itphotosensitive.

[0037] An optical signal of wavelength, λ, and having a firstpolarization corresponding a fast principal state of polarization) and asecond polarization (corresponding a slow principal state ofpolarization) propagates through the optical fiber 160, at the input170, and into the PC 110. The first and second polarizations have a DGD,Δτ, with the second polarization lagging behind the first polarization.The PC 110 aligns the first and the second polarization with the slowand fast principal axes, respectively, of the PM fibers 130, 150. Theoptical signal propagates through the PM fiber 150 to the 3-port opticalcirculator 120. In the preferred embodiment of the invention, the PMfiber 150 is located between the PC and the 3-port optical circulatorand is short enough so that any DGD produced in the FM fiber 150 isnegligible. However, embodiments of the invention are not limited to ashort PM fiber 150. The 3-port optical circulator 120 re-directs theoptical signal into the PM fiber 130. The optical signal propagates adistance through the PM fiber 130 before being reflected due to couplingwith the chirped grating 140. More particularly, the first and secondpolarizations enter the PM fiber 130 with a DGD, Δτ, wherein the firstpolarization enters before the second polarization. The first and secondpolarizations each propagate a respective distance through the PM fiber130 before being reflected through coupling with the chirped grating140. In the PM fiber 130, the first polarization is aligned with theslow principal axis of the PM fiber 130 whereas the second polarizationis aligned with the fast axis of the PM fiber 130. The firstpolarization and the second polarization therefore have different groupvelocities and this introduces a DGD. Furthermore, the firstpolarization and the second polarization are reflected at differentpoints along the length of the PM fiber 130 and this also introduces aDGD. The sum of DGDs introduced in the PM fiber 130 is Δτ′ and thereforethe optical signal emerges from the PM fiber 130 with a total DGD,Δτ₁=Δτ+Δτ′. More particularly, in the embodiment of FIG. 1A, the firstand second polarizations propagate through the PM fiber 130 in a mannerthat the first polarization undergoes a greater time delay in the PMfiber 130 when compared to a time delay, in the PM fiber 130, of thesecond polarization. The sum of DGDs Δτ′ introduced in the PM fiber 130is such that Δτ=−Δτ′, and therefore the optical signal emerges from thePM fiber 130 with the first and second polarizations being Synchronizedwith a total DGD, Δτ₁=Δτ+Δτ′=0. The optical signal is then re-directed,by the 3-port optical circulator 120, to the output 190 through theoptical fiber 180.

[0038] As discussed above, a DGD is introduced in the PM fiber 130 dueto the first and second polarizations being reflected at differentpoints along the PM fiber 130. More particularly, the PM fiber 130reflects the first and second polarizations of wavelength, λ, at twodifferent points along its length where the spatial period, Λ, satisfiesa Bragg condition. The first polarization is aligned with the slowprincipal axis and the Bragg condition is given by Λ=Λ₁=λ/(2n_(s,eff))where n_(s,eff) is an effective index of refraction of the PM fiber 130along the slow principal axis. However, the second polarization isaligned with the fast principal axis and the Bragg condition is given byΛ=Λ₂=λ/(2n_(f,eff)) where n_(f,eff) is an effective index of refractionof the PM fiber 130 along the fast principal axis. n_(s,eff)>n_(f,eff)and therefore Λ₁<Λ₂. This results in the first and second polarizationsbeing reflected at different points along the PM fiber 130. Furthermore,since the spatial period decreases with distance from the end 102 of thePM fiber 130, which is connected to the 3-port optical circulator 120,the first polarization propagates further into the PM fiber 130 than thefirst polarization.

[0039] Embodiments of the invention are not limited to cases in whichthe PM fiber 130 has a negative chirp. In other embodiments of theinvention the PM fiber 130 has a positive chirp in which the spatialperiod, Λ, increases from the end 102 gradually along the length of thePM fiber 130. Furthermore, embodiments of the invention are not limitedto a quadratic chirp and in other embodiments of the invention the PMfiber 130 has a linear or non-linear chirp. However, in embodiments ofthe invention wherein the PM fiber 130 has negative linear chirp, thefirst and second polarizations are aligned with the slow and fastprincipal axes, respectively, of the PM fiber 130, whereas, inembodiments of the invention wherein the PM fiber 130 has positivelinear chirp, the first and second polarizations are aligned with thefast and slow principal axes, respectively, of the PM fiber 130.

[0040] In the above example, the optical signal is a monochromaticoptical signal of wavelength, λ. However, the chirped grating 140 allowsfor a continuous range of wavelengths, λ_(j) wherein λ_(b)≦λ≦λ_(a), tobe reflected and embodiments of the invention are not limited toperforming PMD compensation of monochromatic signals. In otherembodiments of the invention, the PM fiber 130 is used to perform PMDCfor an optical signal having dispersion wherein the optical signal has acontinuous range of wavelengths, λ_(j), centered about a centerwavelength. In such embodiments, respective first and secondpolarizations of the wavelengths, λ_(j), have respective DGDs which maybe different from one another resulting in a DGD, Δτ, given by Δτ=<Δτ>where <Δτ> is an average DGD. In such cases, the PM fiber 130 is tunedto introduce a DGD Δτ′, given by Δτ′=<Δτ′> where <Δτ′> an average DGD,so that PMD compensation is produced resulting in a total DGD,Δτ₁=<Δτ₁>=<Δτ>+<Δτ′>=0 where <Δτ₁> is a total average DGD. A method fortuning the FM fiber 130 will now be described.

[0041] Referring to FIG. 2, shown is a schematic block diagram of a PMDcompensator 200, in another embodiment of the invention. The PMDcompensator 200 of FIG. 2 is similar to the PMDC 100 of FIG. 1 exceptthat in FIG. 2 the PMDC 200 has an optical tap 210 connected to the3-port optical circulator 120, a piezo-electric device 220, in which thePM fiber 130 is embedded, and a control circuit 230 connected to the PC110, to the optical tap 210 and to the piezo-electric device 220. Anoptical signal, at the input 170, has a first polarization and a secondpolarization lagging the first polarization. After having propagatedthrough the PC 110, the 3-port optical circulator 120, the FM fiber 130and back through the 3-port optical circulator 120, the optical signalpropagates through the optical tap 210. A major portion of the opticalsignal is then output to the output 190 through the optical fiber 180and a minor portion of the optical signal propagates to the controlcircuit 230. The control circuit 230 detects the minor portion of theoptical signal. The control circuit 230 measures a polarization state ofthe minor portion of the optical signal and the total DGD, Δτ₁, of thefirst and second polarizations from the minor portion of the opticalsignal. The control circuit 230 then provides instructions to the PC 110for controlling the alignment of the first and second polarizations withthe slow and fast axes, respectively, of the PM fiber 130 based oninformation on the polarization state. The control circuit 230 alsoprovides instructions to the piezo-electric device for applying atensile force to stretch the PM fiber 130, based on information on thetotal DGD, Δτ₁. By stretching the PM fiber 130, the piezo-electricdevice 220 controls the spatial period, Λ, of the chirped grating 140 ina manner that allows the point at which the optical signal is reflectedto be controlled. In other embodiments of the invention, thepiezo-electric device 220 is replaced by one or more heaters. In suchembodiments of the invention, the heaters control n_(s,eff) andn_(f,eff) in a manner that allows the point at which the optical signalis reflected in the PM fiber 130 to be controlled. Possible tuningmechanisms for the PM fiber 130 include, but are not limited to heatingand to stretching using a piezo-electric device.

[0042] Referring to FIG. 3, shown is a schematic block diagram of a PMDcompensator 300, in another embodiment of the invention. The PMDcompensator 300 includes the PC 110, the 3-port optical circulator 120and the PM fiber 130 having the chirped grating 140. The PC 110 isconnected to the PM fiber 330. The PC 110 is also connected to the3-port optical circulator 120 through an optical fiber 310.

[0043] An optical signal, at the input 170, has a first polarization anda second polarization lagging the first polarization with a DGD, Δτ. Theoptical signal propagates through the optical fiber 160, at the input170, and into the 3-port optical circulator 120 where it is re-directedinto the PC 110. The PC 110 aligns the first and second polarizationswith the slow and fast principal axes of the PM fiber 130, respectively.The optical signal then propagates into the PM fiber 130 where it isreflected and undergoes PMD compensation. The optical signal thenpropagates back into the PC 110 to the 3-port optical circulator 120where it is re-directed into the optical fiber 180 at the output 190.

[0044] Referring to FIG. 4, shown is a schematic block diagram of a PMDcompensator 400, in another embodiment of the invention. The PMDcompensator 400 of FIG. 4 is similar to the PMD compensator 300 of FIG.3 except that in FIG. 4 the PMD compensator 400 has an optical tap 410connected to the 3-port optical circulator 120, a piezo-electric device420, in which the PM fiber 130 is embedded, and a control circuit 430.The control circuit 430 is connected to the optical tap 410, to thepiezo-electric device 420 and to the PC 110. After undergoing PMDCthrough the PM fiber 130 an optical signal propagates back through thePC 110, into the optical circulator 120 and into the optical tap 410. Amajor portion of the optical signal is then output through the opticalfiber 180 to the output 190 and a minor portion of the optical signalpropagates to the control circuit 430. The control circuit 430 detectsthe minor portion of the optical signal. The control circuit 430 alsomeasures a polarization state of the minor portion of the optical signaland measures the total DGD, Δτ₁, of the first and second polarizationsfrom the minor portion of the optical signal. The control circuit 430then provides instructions to the PC 110 for controlling the alignmentof the first and second polarizations with the slow and fast axes,respectively, of the PM fiber 130 based on information on thepolarization state. The control circuit 430 also provides instructionsto the piezo-electric device 420 for applying a tensile force to stretchthe PM fiber 130, based on information on the total DGD, Δτ₁. In otherembodiments of the invention, the piezo-electric device 420 is replacedby one or more heaters. Possible tuning mechanisms for the PM fiber 130include, but are not limited to heating and to stretching using apiezo-electric device.

[0045] The PMD compensators 100, 200, 300, 400 of FIGS. 1A and 2 to 4are used to perform PMDC of monochromatic and dispersive opticalsignals. For a dispersive optical signal of wavelengths, λ_(j), thepoints at which first and second polarizations of the wavelengths,λ_(j), are reflected depend on the Bragg condition which, in turn,depends on the wavelengths, λ_(j). The PM fiber 130 therefore changesthe dispersion of the dispersive optical signal. Consequently, in someembodiments of the invention, one of a positive chirp and a negativechirp which results in a decrease in the dispersion of the dispersiveoptical signal is used to perform partial CDC (Chromatic DispersionCompensation) Furthermore, in some embodiments of the invention, anoptical fiber with a chirped grating is combined with any one of the PMDcompensators 100, 200, 300, 400 of FIGS. 1A and 2 to 4 to performfurther CDC of the dispersive optical signal in addition to performingPMDC.

[0046] Referring to FIG. 5, shown is a schematic block diagram of a PMDand CD compensator 500, in another embodiment of the invention. The PMDand CD compensator 500 includes the PC 110, a 4-port optical circulator520, the PM fiber 130 with the chirped grating 140, and an optical fiber530 with a chirped grating 540. The PC 110 is connected to the PM fiber130 through the PM fiber 150 and the 4-port optical circulator 520. ThePM fibers 130, 150 are aligned so that the respective fast principalaxes of the two PM fibers 130, 150 are aligned and so that therespective slow principal axes of the two PM fibers 130, 150 arealigned. The optical fiber 530 is connected to the 4-port opticalcirculator 520.

[0047] At the input 170, an optical signal has PMD and CD. Moreparticularly, the optical signal is dispersive having continuous rangeof wavelengths, λ_(j), wherein λ_(b)≦λ_(j)≦λ_(a) with each wavelengthhaving a first polarization and a second polarization lagging the firstpolarization resulting in a DGD, Δτ=<Δτ>. The optical signal propagatesthrough the PC 110 and is re-directed into the PM fiber 130, through the4-port optical circulator 520, where it is reflected. As discussedabove, different group velocities and different points of reflection inthe PM fiber 130 of the first and second polarizations of thewavelengths, λ_(j), result in a reduction in the DGD, Δτ=<Δτ>, thusresulting in PMD compensation. The optical signal then propagatesthrough the 4-port optical circulator 520 where it is re-directed intothe optical fiber 530. A waveform associated with the optical signal isslightly distorted due to dispersion effects and the wavelengths, λ_(j),with λ_(b)≦λ_(j)≦λ_(a) of the optical signal enter sequentially, intime, into the optical fiber 530. Each wavelength, λ_(j), propagatesrespective a length into the optical fiber 530 before being reflectedback into the 4-port optical circulator 520. More particularly, each oneof the wavelengths, λ_(j), is reflected at a respective point along theoptical fiber 530 where a spatial period, Λ′ of the chirped grating 540satisfies Λ′=Λ′_(j)=λ_(j)/(2n′_(eff)) where Λ′_(j) is a spatial periodand n′_(eff) is an effective index of refraction of the optical fiber530. Each one of the wavelengths, λ_(j), therefore travels through aspecific optical path length that is controlled by a slope, length andnon-linearity of the chirped grating 540. The wavelengths, λ_(j), eachperform a round trip through the optical fiber 530 and they exit theoptical fiber 530 in synchronization. The optical signal then propagatesback into the 4-port optical circulator 520 where it is re-directed outinto the optical fiber 180 to the output 190.

[0048] Referring to FIG. 6, shown is a schematic block diagram of a PMDand CD compensator 600, in another embodiment of the intention. The PMDand CD compensator 600 of FIG. 6 is similar to the PMD and CDcompensator 500 of FIG. 5 except that in FIG. 6 the PMD and CDcompensator 600 has an optical tap 610 connected to the 4-port opticalcirculator 520, a piezo-electric device 620 in which the PM fiber 130 isembedded, a piezo-electric device 621 in which the optical fiber 530 isembedded, and a control circuit 630. The control circuit 630 isconnected to the optical tap 610, to the piezo-electric devices 620, 621and to the PC 110. After undergoing PMDC through the PM fiber 130 andundergoing CDC through the optical fiber 530 a dispersive optical signalpropagates through the 4-port optical circulator 520 and into theoptical tap 610. A major portion of the optical signal is then outputthrough the optical fiber 180 to the output 190 and a minor portion ofthe optical signal propagates to the control circuit 630. The controlcircuit 630 detects the minor portion of the optical signal. The controlcircuit 630 also measures the dispersion and the polarization state ofthe minor portion of the optical signal and measures the total averageDGD, <Δτ₁>, of the first and second polarizations from the minor portionof the optical signal. The control circuit 630 then providesinstructions to the PC 110 for controlling the alignment of the firstand second polarizations with the slow and fast axes, respectively, ofthe PM fiber 130 based on information on the polarization state. Thecontrol circuit 630 provides instructions, based on information on theaverage total average DGD, <Δτ₁>, to the piezo-electric device 620, inwhich the PM fiber 130 is embedded, for stretching the PM fiber 130. Thecontrol circuit 630 also provides instructions, based on information onthe dispersion, to the piezo-electric device 621, in which the opticalfiber 530 is embedded, for stretching the optical fiber 530. In otherembodiments of the invention, the piezo-electric device 621 is replacedby one or more heaters. In such embodiments, the heaters apply heat tothe optical fiber 530, based on the dispersion, to tune the effectiveindex of refraction n′_(eff) of the optical fiber and reduce thedispersion. Furthermore, in some embodiments of the invention, thepiezo-electric device 620 is replaced by one or more heaters.

[0049] Referring to FIG. 7, shown is a schematic block diagram of a PMDand CD compensator 700, in another embodiment of the invention. The PMDand CD compensator 700 includes the PC 110, the 4-port opticalcirculator 520, the PM fiber 130 with the chirped grating 140 and theoptical fiber 530 with the chirped grating 540. The PC 110 is connectedto the PM fiber 130 and to the 4-port optical circulator 520 through anoptical fiber 789 whereas thee optical fiber 530 with the chirpedgrating 540 is connected to the 4-port optical circulator 520. At theinput 170, an optical signal has PMD and CD. More particularly, theoptical signal has a continuous range of wavelengths, λ_(j), each havinga first polarization and a second polarization lagging the firstpolarization resulting in the average DGD, <Δτ>. The PC 110 aligns thefirst and second polarizations with the slow and fast principal axes,respectively, of the PM fiber 130. The optical signal then propagatesinto the PM fiber 130 where it is reflected and undergoes PMDcompensation. The optical signal then propagates back into the PC 110 tothe 4-port optical circulator 520 where it is re-directed into theoptical fiber 530. Within the optical fiber 530 the optical signal isreflected and undergoes CD compensation. The optical signal then emergesfrom the optical fiber 530 and propagates into the 4-port opticalcirculator 520 where it is re-directed into the optical fiber 180 to theoutput 190.

[0050] Referring to FIG. 8, shown is a schematic block diagram of a PMDand CD compensator 800, in yet another embodiment of the invention. ThePMD and CD compensator 800 of FIG. 8 is similar to the PMD and CDcompensator 700 of FIG. 7 except that in FIG. 8 the PMD and CDcompensator 800 has an optical tap 810 connected to the 4-port opticalcirculator 520, a piezo-electric device 820 in which the PM fiber 130 isembedded, a piezo-electric device 821 in which the optical fiber 530 isembedded, and a control circuit 830. The control circuit 830 isconnected to the optical tap 810, to the piezo-electric devices 820, 821and to the PC 110. After undergoing CDC through the optical fiber 130 adispersive optical signal propagates through the 4-port opticalcirculator 520 and into the optical tap 810. A major portion of theoptical signal is then output through the optical fiber 180 to theoutput 190 and a minor portion of the optical signal propagates to thecontrol circuit 830. The control circuit 830 detects the minor portionof the optical signal. The control circuit 830 also measures thedispersion and the polarization state of the minor portion of theoptical signal and measures the total average DGD, <Δτ₁>, of the firstand second polarizations from the minor portion of the optical signal.The control circuit 830 then provides instructions to the PC 110 forcontrolling the alignment of the first and second polarizations with theslow and fast axes, respectively, of the PM fiber 130 based oninformation on the polarization state. The control circuit 830 providesinstructions, based on information on the total average DGD, <Δτ₁>, tothe piezo-electric device 820, in which the PM fiber 130 is embedded,for stretching the PM fiber 130. The control circuit 830 also providesinstructions, based on information on the dispersion, to thepiezo-electric device 821, in which the optical fiber 530 is embedded,for stretching the optical fiber 530. In other embodiments of theinvention, at least one of the piezo-electric devices 820, 821 arereplaced by one or more heaters.

[0051] In effect, the control circuit 830, the piezo-electric devices820, 821 and the optical tap 810, in conjunction with the PC 110, the PMfiber 130 and the optical fiber 530, form a control system forcontrolling alignment of the first and second polarization andcontrolling the dispersion.

[0052] Referring to FIG. 9, shown is a schematic block diagram of a PMDand CD compensator 900, in yet another embodiment of the invention. ThePMD and CD compensator 900 of FIG. 9 is similar to the PMD and CDcompensator 500 of FIG. 5 except that the 4-port optical circulator 520of FIG. 5 is replaced with a 5-port optical circulator 920 and anotheroptical fiber 930 with a chirped grating 940 is connected to the 5-portoptical circulator 920. In such an embodiment, an optical signalundergoes CDC in both optical fibers 530, 930. More particularly, one ofthe chirped gratings 540, 940 is a positive chirped grating and theother one is a negative chirped grating. A combination of a positive anda negative chirp allows the optical fibers 530, 930 to collectivelyprevent introduction of second order chromatic dispersion effects duringDCD.

[0053] In some embodiments of the invention the PMD and CD compensator900 is equipped with a control system, as discussed above with referenceto FIG. 8, for controlling alignment of the first and secondpolarization and controlling the dispersion.

[0054] Embodiments of FIGS. 1 to 9, are shown with the PM fibers 130,150 and optical fibers 530, 930, 160, 180. However, embodiments of theinvention are not limited to PM fibers and optical fibers. In otherembodiments the invention, the optical fibers 530, 930, 160, 180 areoptical wave-guides made of any suitable material capable oftransmitting light and capable of performing wave-guide functionality.More particularly, in sore embodiments, the optical wave-guides areoptical planar wave-guides. Furthermore, in some embodiments of theinvention, the PM fibers 130, 150 are made of any suitable birefringentmaterial, having a slow principal axis with index of refraction,n_(s,eff) and a fast principal axis with index of refraction, n_(f,eff),capable of transmitting light and capable of performing wave-guidefunctionality. Such a birefringent material is referred to as abirefringent wave-guide. More particularly, in some embodiments, thebirefringent wave-guides are birefringent planar wave-guides. Note thatPM fibers and optical fibers form subsets of wave-guides.

[0055] A chirped grating is impressed on to the wave-guides using anyone of the methods discussed above with regards to the PM fiber 130 ofFIG. 1. Furthermore, in some embodiments of the invention, a chirpedgrating is impressed on to the wave-guides by etching the wave-guideswith the spatial period, Λ.

[0056] In some embodiments the wave-guides are integrated on a chip.Furthermore in some embodiments, the optical circulators 120, 520, 920of FIGS. 1 to 9 are also integrated on an chip. Finally, in someembodiments, functions of the optical taps 21 0, 410, 610, 810 and thecontrol circuits 230, 430, 630, 830 of FIGS. 2, 4, 6 and 8, respectivelyare also integrated on a chip,

[0057] Numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

We claim:
 1. An optical apparatus comprising: a birefringent wave-guideadapted to receive an optical signal having a first polarization and asecond polarization and adapted to allow the first polarization and thesecond polarization to propagate at different group velocities, thebirefringent wave-guide comprising: a chirped grating adapted to reflectthe first polarization and the second polarization at different pointsalong the birefringent wave-guide.
 2. An optical apparatus according toclaim 1 wherein the birefringent wave-guide is a birefringent planarwave-guide.
 3. An optical apparatus according to claim 1 wherein thebirefringent wave-guide is a PM (Polarization Maintaining) fiber.
 4. Anoptical apparatus adapted to perform PMDC (Polarization Mode DispersionCompensation), the apparatus comprising: a birefringent wave-guidecomprising a fast principal axis, a slow principal axis and a chirpedgrating; and a PC (Polarization Controller) connected to thebirefringent wave-guide and adapted to receive an optical signal havinga first polarization and a second polarization that lags the firstpolarization with a DGD (Differential Group Delay), Δτ, the PC alsobeing adapted to align the first polarization with one of the slowprincipal axis and the fast principal axis and to align the secondpolarization with another one of the slow principal axis and the fastprincipal axis, so that the first polarization and the secondpolarization propagate at different group velocities through thebirefringent wave-guide and are reflected, through coupling with thechirped grating, at different points along the birefringent wave-guideresulting in the first polarization undergoing a greater time delay inthe birefringent wave-guide when compared to a time delay, in thebirefringent wave-guide, of the second polarization.
 5. An opticalapparatus according to claim 4 wherein the birefringent wave-guide is aplanar wave-guide.
 6. An optical apparatus according to claim 4 whereinthe birefringent wave-guide is a PM fiber.
 7. An optical apparatusaccording to claim 4 comprising an optical circulatory connected to thePC and adapted to re-direct the optical signal propagating from an inputinto the PC and re-direct the optical signal propagating from the PC toan output.
 8. An optical apparatus according to claim 7 comprising meansfor tuning a total DGD, Δτ₁=Δτ+Δτ′, at the output, wherein Δτ′ is a DGDintroduced in the birefringent wave-guide.
 9. An optical apparatusaccording to claim 4 wherein the birefringent wave-guide is adapted toperform PMDC and CDC (Chromatic Dispersion Compensation) of a dispersiveoptical signal having wavelengths, λ_(j), and an average DGD, <Δτ>. 10.An optical apparatus according to claim 7 adapted to perform PMDC andCDC of a dispersive optical signal having wavelengths, λ_(j), and anaverage DGO, <Δτ>, the apparatus comprising an optical wave-guide havinga chirped grating connected to the optical circulator, the opticalwave-guide being adapted to receive the dispersive optical signal andreflect the wavelengths, λ_(j), at different points along the opticalwave-guide in a manner that the wavelengths, λ_(j), emerge from theoptical wave-guide in synchronization.
 11. An optical apparatusaccording to claim 10 wherein the optical wave-guide is an opticalfiber.
 12. An optical apparatus according to claim 10 comprising controlmeans for tuning a total average DGD, <Δτ₁>=<Δτ>+<Δτ′>, of thedispersive optical signal, detected at the output, wherein <Δτ′> is anaverage DGD introduced in the birefringent wave-guide.
 13. An opticalapparatus according to claim 10 comprising control means for tuning thedispersion of the dispersive optical signal detected at the output. 14.An optical apparatus adapted to perform PMDC, the apparatus comprising;a birefringent wave-guide comprising a fast principal axis, a slowprincipal axis and a chirped grazing; an optical circulator; and a PCconnected to the birefringent wave-guide, through the opticalcirculator, and adapted to receive an optical signal having a firstpolarization and a second polarization that lags the first polarizationwith a DGD, Δτ, the PC also being adapted to align the firstpolarization with one of the slow principal axis and the fast principalaxis and to align the second polarization with another one of the slowprincipal axis and the fast principal axis, so that the firstpolarization and the second polarization propagate at different groupvelocities through the birefringent wave-guide and are reflected,through coupling with the chirped grating, at different points along thebirefringent wave-guide resulting in the first polarization undergoing agreater time delay in the birefringent wave-guide when compared to atime delay, in the birefringent wave-guide, of the second polarization.15. An optical apparatus according to claim 14 wherein the birefringentwave-guide is a birefringent planar wave-guide.
 16. An optical apparatusaccording to claim 14 wherein the birefringent wave-guide is a PM fiber.17. An optical apparatus according to claim 14 wherein the chirpedgrating is one of a positive chirped grating and a negative chirpedgrating.
 18. An optical apparatus according to claim 14 wherein thechirped grating has one of a linear spatial period, a non-linear spatialperiod and a quadratic spatial period.
 19. An optical apparatusaccording to claim 14 comprising control means for tuning a total DGD,Δτ₁=Δτ+Δτ′, of the optical signal at an output, wherein Δτ′ is a DGDintroduced in the birefringent wave-guide.
 20. An optical apparatusaccording to claim 14 wherein the birefringent wave-guide is adapted toperform PMDC and CDC (Chromatic Dispersion Compensation) of a dispersiveoptical signal having wavelengths, λ_(j), and an average DGD, <Δτ>. 21.An optical apparatus according to claim 14 comprising: a piezo-electricdevice, in which the birefringent wave-guide is embedded, adapted tocontrol a spatial period, Λ, of the chirped grating; an optical tap, atan output of the optical circulator, adapted to redirect a minor portionof the optical signal; and a control circuit connected to the opticaltap, the piezo-electric device and the PC, wherein the control circuitis adapted to receive the minor portion of the optical signal, detect apolarization state of the minor portion of the optical signal and detecta total DGD, Δτ₁=Δτ+Δτ′, of the first and second polarizations from theminor portion of the optical signal, wherein Δτ′ is a DGD introduced inthe birefringent wave-guide, the control circuit also being adapted toprovide instructions to the piezo-electric device for stretching thebirefringent wave-guide, based on the total DGD, Δτ₁, and to provideinstructions to the PC for tuning an alignment of the first and secondpolarizations with a respective one of the slow and fast principal axesof the birefringent wave-guide, based on the polarization state.
 22. Anoptical Apparatus according to claim 14 comprising: one or more heaters,in which the birefringent wave-guide is embedded, adapted to controleffective indexes of refraction, n_(s,eff), n_(f,eff), of the slow andfast principal axes, respectively, of the birefringent wave-guide; anoptical tap, at an output of the optical circulator, adapted to redirecta minor portion of the optical signal; and a control circuit connectedto the optical tap, the piezo-electric device and the PC, wherein thecontrol circuit is adapted to receive the minor portion of the opticalsignal, detect a polarization state of the minor portion of the opticalsignal and detect a total DGD, Δτ₁=Δτ+Δτ′, of the first and secondpolarizations from the minor portion of the optical signal, wherein Δτ′is a DGD introduced in the birefringent wave-guide, the control circuitalso being adapted to provide instructions to the one or more heatersfor tuning the effective indexes of refraction, n_(s,eff), n_(f,eff),based on the total DGD, Δτ₁, and to provide instructions to the PC fortuning an alignment of the first and second polarizations with arespective one of the slow and fast principal axes of the birefringentwave-guide, based on the polarization state.
 23. An optical apparatusaccording to claim 14 comprising an another wave-guide connecting the PCwith the optical circulator.
 24. An optical apparatus according to claim14 wherein the optical circulator is a 3-port optical circulator.
 25. Anoptical apparatus according to claim 14 adapted to perform PMDC and CDCof a dispersive optical signal having wavelengths, λ_(j), the apparatuscomprising an optical wave-guide having a chirped grating connected tothe optical circulator, the optical wave-guide being adapted to receivethe dispersive optical signal and reflect the wavelengths, λ_(j), atdifferent points along the optical wave-guide in a manner that thewavelengths, λ_(j), emerge from the optical wave-guide insynchronization.
 26. An optical apparatus according to claim 25 whereinthe optical wave-guide is an optical fiber.
 27. An optical. apparatusaccording to claim 25 wherein the optical circulator is a 4-port opticalcirculator.
 28. An optical apparatus according to claim 25 comprisingcontrol means for tuning an total average DGD, <Δτ₁>=<Δτ>+<Δτ′>, of thedispersive optical signal detected at an output, wherein <Δτ′> is a DGDintroduced in the birefringent wave-guide.
 29. An optical apparatusaccording to claim 25 comprising control means for tuning a dispersionof the dispersive optical signal detected at an output.
 30. An opticalapparatus according to claim 25 comprising: a piezo-electric device, inwhich the optical wave-guide is embedded, adapted to control a spatialperiod, Λ′, of the chirped grating of the optical wave-guide; an opticaltap, at an output of the optical circulator, adapted to redirect a minorportion of the dispersive optical signal; and a control circuitconnected to the optical tap and the piezo-electric device, wherein thecontrol circuit is adapted to receive the minor portion of thedispersive optical signal, detect a dispersion of the minor portion ofthe dispersive optical signal and provide instructions to thepiezo-electric device for stretching the optical wave-guide, based onthe dispersion of the minor portion of the dispersive optical signal.31. An optical apparatus according to claim 25 comprising: one or moreheaters, in which the optical wave-guide is embedded, adapted to controlan effective index of refraction, n′_(eff), of the optical wave-guide;an optical tap, at an output of the optical circulator, adapted toredirect a minor portion of the dispersive optical signal; and a controlcircuit connected to the optical tap and the piezo-electric device,wherein the control circuit is adapted to receive the minor portion ofthe dispersive optical signal, detect a dispersion of the minor portionof the dispersive optical signal and to provide instructions to the oneor more heaters for tuning the effective index of refraction, n′_(eff),based on the dispersion of the minor portion of the dispersive opticalsignal.
 32. An optical apparatus according to claim 25 comprising twooptical wave-guides each having a chirped grating wherein one of the twooptical wave-guides has a positive chirped grating and another one ofthe two optical wave-guides has negative chirped grating, the twooptical wave-guides being connected to the optical circulator and beingadapted to collectively perform CDC and prevent introduction of secondorder chromatic dispersion effects during CDC.
 33. An integrated chipcomprising the optical apparatus of claim 14 wherein the birefringentwave-guide, the optical circulator and the PC are implemented on themicrochip.
 34. An optical transmission system comprising an opticalapparatus according to claim
 14. 35. A method of performing PMDC upon anoptical signal having a first polarization and a second polarization,wherein the second polarization lags the first polarization with a DGD,Δτ; the method comprising: aligning the first polarization with one of aslow principal axis and a fast principal axis of a birefringentwaveguide having a chirped grating and aligning the second polarizationwith another one of the slow principal axis and the fast principal axis;and propagating the first polarization and the second polarizationthrough the birefringent waveguide at different group velocities andreflecting the first polarization and the second polarization atdifferent points along the birefringent waveguide; wherein the aligningthe first polarization and the aligning the second polarization areperformed in a manner that the first polarization undergoes a greatertime delay in the birefringent waveguide when compared to a time delay,in the birefringent waveguide, of the second polarization.
 36. A methodaccording to claim 35 comprising re-directing the optical signal to anoutput when the optical signal emerges from the birefringent wave-guideafter being reflected.
 37. A method according to claim 36 comprising:measuring a polarization state of a minor portion of the optical signalat the output; and tuning an alignment of the first polarization and thesecond polarization with a respective one of the slow principal axis andthe fast principal axis based on the polarization state.
 38. A methodaccording to claim 36 comprising; measuring a total DGD, Δτ₁=Δτ+Δτ′,between the first polarization and the second polarization, at theoutput, wherein Δτ′ is a DGD introduced in the birefringent wave-guide;and tuning a spatial period, Λ, of the chirped grating, based on thetotal DGD, Δτ₁, by applying a tensile force upon the birefringentwave-guide to stretch the birefringent wave-guide and reduce the totalOGD, Δτ₁.
 39. A method ac(cording to claim 36 comprising: measuring atotal DGD, Δτ₁=Δτ+Δτ′, between the first polarization and the secondpolarization, at the output, wherein Δτ′ is a DGD introduced in thebirefringent wave-guide; and applying heat to the birefringentwave-guide to tune effective indexes of refraction, n_(s,eff),n_(f,eff), of the slow and fast principal axes, respectively, forreducing the total DGD, Δτ₁.
 40. A method of performing PMDC and CDCcomprising the method of claim 35, wherein the optical signal is adispersive optical signal having wavelengths, λ_(j), and wherein afterthe propagating the first polarization and the second polarizationthrough the birefringent wave-guide at different group velocities andreflecting the first polarization and the second polarization atdifferent points along the birefringent wave-guide, the methodcomprising: propagating the dispersive optical signal, and reflectingthe wavelengths, λ_(j), at different points along an optical wave-guidein a manner that the wavelengths, λ_(j), emerge from the opticalwave-guide in synchronization after being reflected.
 41. A methodaccording to claim 40 comprising re-directing the dispersive opticalsignal to an output when it emerges from the optical wave-guide.
 42. Amethod according to claim 41 comprising: measuring the dispersion of thedispersive optical signal at the output; and tuning a spatial period,Λ′, of the chirped grating of the optical wave-guide, based on thedispersion, by applying a tensile force upon the optical wave-guide tostretch the optical wave-guide.
 43. A method according to claim 41comprising: measuring the dispersion of the dispersive optical signal atthe output.; and applying heat to the optical wave-guide, based on thedispersion, to tune an effective index of refraction n′_(eff) of theoptical wave-guide and reduce the dispersion.